An indicator is a halochromic chemical compound that is added to a solution to determine its pH. It is a chemical detector for hydronium ions. The indicator changes colour to show the pH of the solution. Some indicates are not very precise and only tell us whether the solution is acidic or basic. When the indicator is added to the solution, they bind to hydrogen or hydroxide ions. The different electron configurations of the bound indicator cause the indicator's colour to change.
Some common indicators are: universal indicator, phenolphthalein, methyl orange, litmus, bromothymol blue.
However, there are some indicators that are found in nature in the form of plant pigments known as anthocyanins, which change colour over different pH ranges, depending on source. For example red cabbage juice will change colour to indicate pH if the solution is within the range of pH 1 - pH 12. Red beet juice will change from red to yellow between pH of 11 and 12. Therefore they are not as reliable as common laboratory indicators. Some other natural indicators include: Carrots, cherries, grapes, hydrangea, onions, poppy petals, rhubarbs, thyme and tulip petals.
To investigate the uses of various common and natural indicators in acidic, neutral and basic solutions.
Part A: COMMON INDICATORS
0.1 M solutions of sodium hydroxide and hydrochloric acid, distilled water, 3 test tubes, test-tube rack, 3 x 100mL beakers, liquid litmus, universal indicator, methyl orange,
1.Using beakers, pour 2cm of acid into one test tube, 2cm of sodium hydroxide into another test tube, and 2cm of distilled water into the third test tube.
2.Add 3 drops of red litmus to each tube. Record results.
3.Repeat steps 2 and 3 for other indicators.
Colour in strong acid (hydrochloric acid)Colour in strong base (sodium hydroxide)Colour in neutral solution (water)
Universal IndicatorDeep redDeep violetYellow
Methyl orangeOrange-redYellowOrange yellow
Bromothymol blueYellowBlueDark Blue green
Distilled water was used instead of tap water because tap water is not completely pure. It may have traces of minerals and chlorine, potassium and sodium in it. This may affect the end result of the experiment.
I found that some indicators distinguish between the acidity and basicity of the solution more obviously then others. For example, the litmus clearly indicates that a acid was peach, a base was blue and a neutral solution was purple, whereas a methyl orange indicator shows that an acid is orange-red and a neutral solution is orange. These two colours are very similar, Also some of the colours that I observed were not the correct, for example the litmus indicator should actually show that an acidic solution is red not peach. I have learnt to wash my test tubes more carefully and not put sodium hydroxide into a test tube that had HCL in it, even...